Etch-back screening

ABSTRACT

A shadow mask is initially prepared with apertures dimensioned for photoresist screening of a color picture tube and, after screening has been accomplished, the mask is re-etched to enlarge the apertures to a desired size in relation to the phosphor deposits. An improvement in the re-etch process is achieved by utilizing a substantially saturated solution of the etchant in the re-etch step.

United States Patent [1 1 Holowka et al.

[ Nov. 13, 1973 ETCH-BACK SCREENING Inventors: Jaroslaw A. Holowka, Chicago;

Martin L. Lerner, River Forest, both of I11.

Assignee: Zenith Radio Corporation, Chicago,

Filed: Apr. 1, 1971 Appl. No.: 130,121

U.S. Cl 156/18, 156/2, l17/5.5, 96/36.l, 96/38, 313/86 Int. Cl. C23g 1/08, C23f 1/02 Field of Search 156/5, 8, 18, 345; 96/38 References Cited UNITED STATES PATENTS 3/1970 White 156/5 2,961,314 11/1960 Amdursky et a]. 96/38 Primary ExaminerJacob H. Steinberg Att0rneyJohn J. Pederson and Cornelius J. OConnor [57] ABSTRACT A shadow mask is initially prepared with apertures dimensioned for photoresist screening of a color picture tube and, after screening has been accomplished, the mask is re-etched to enlarge the apertures to a desired size in relation to the phosphor deposits. An improvement in the re-etch process is achieved by utilizing a substantially saturated solution of the etchant in the re-etch step.

5 Claims, 4 Drawing Figures Etchonr ETCH-BACK SCREENING BACKGROUND OF THE INVENTION The invention is particularly concerned with screening of a color picture tube characterized by the fact that the phosphor deposits are smaller than the apertures of the color-selection electrode. Typical of this kind of tube is that which is known as a black surround picture tube as well as the post-deflection-focus tube. In both cases, the tube may come in different sizes and configurations, being either rectangular or round. Additionally, the phosphor pattern, which consists of interleaved deposits of various phosphor materials, may be arranged in the form of dot triads or elongated stripes. The present invention is not particularly affected by the choice made of these parameters and, for convenience, it will be assumed that the tube in process is rectangular and has dots of red, green and blue phosphor materials arranged to define a multiplicity of dot triads disposed throughout a rectangularly shaped image field.

Tubes of the type under consideration differ from the more conventional shadow mask tubes, for example, in that the phosphor deposits are reduced in size. In the conventional tube the phosphor dots are in tangential contact with one another, whereas in the blacksurround tube to which this invention is especially directed the reduced phosphor dots are separated from one another. Their separation provides elemental portions of the screen surface to which a black pigment or other light-absorbing material may be applied, surrounding each phosphor with the light-absorbing material and giving rise to its name black surround tube. In its preferred form, the phosphor dots are smaller in diameter than the holes of the shadow mask in order that the cross sectional area of the electron beams will be larger than the phosphor dots to provide a guard band or tolerance for color purity in the picture tube. A tube of this description is the subject of U.S. Pat. No. 3,146,368 issued Aug. 25, 1964 in the name of Fiore et al. and assigned to the assignee of the present invention. It has vastly improved brightness and contrast properties.

The post-deflection-focus tube is generally the same except that it may or may not include black surround material. It requires that the apertures of the mask be larger than the phosphor dots because it has an electron focusing system which focuses the electron beams after the plane of deflection to the end that more electrons of the beams are able to reach the screen than heretofore. This tube also has enhanced brightness.

In the manufacture of tubes of the type under consideration difficulty has been experienced in obtaining the necessary dimensional relation ofphosphor dots to electron beam size or cross sectional diameter. An effective solution to the problem which is successful in the commercial production of black-surround tubes has been the re-etch or etch-back process. In accordance with that process the mask has a field of apertures that are etched initially to a predetermined size for use in photoresist printing of the screen. This is a well-known printing process which takes advantage of the property of a photosensitive resist that, upon exposure to actinic energy, it changes its solubility in a particular solvent. The most attractive embodiment of the process uses a water-based system comprising an aqueous solution of sensitized polyvinyl alcohol which is rendered insoluble in water when exposed to ultraviolet light. After the three phosphor materials have been applied to the screen area in locations precisely determined by exposure through the shadow mask of the color picture tube, that mask is then subjected to a second etching process in order to enlarge the apertures to a size greater than that which is used for screening. Although this process has been and is in successful commercial use, the present invention is an attractive improvement in that it permits the etch-back or re-etch step to effect a minimal change in the thickness of the shadow mask thereby preserving its mechanical strength. This, of course, is most desirable because it greatly reduces losses that might otherwise be experienced as an incident to handling the mask in fabricating black-surround tubes.

Accordingly, it is an object of the invention to improve the etching process by which apertures of a color-selection electrode or shadow mask are enlarged a desired amount.

It is a particular object of the invention to improve the re-etch or etch-back process of color tube screening to enhance the mechanical strength of the colorselection electrode or shadow mask in its final form.

It is a very particular object of the invention to improve the re-etch process of screening a shadow mask color tube to minimize the reduction in wall thickness of the shadow mask.

SUMMARY OF THE INVENTION This invention applies most particularly to the manufacture of a color cathode-ray tube wherein an interleaved pattern of various phosphor materials is screened on the image area by a photomechanical printing process in which that area is exposed to actinic energy through the apertures of a metallic colorselection electrode which is soluble in a given solvent. The improvement of the invention comprises enlarging the apertures of the electrode, after screening the image area with phosphor deposits, by applying to the barren metallic surfaces of the electrode a substantially saturated solution of the etchant.

As practiced today, the shadow mask is cold rolled steel or an alloy that is soluble in ferric chloride. Reetching is accomplished by spraying the shadow mask with an aqueous etching solution having a Baume of approximately 49" Further improvement may be realized, if desired, by including a minor amount of free hydrochloric acid in the etchant as a solvent for ferrous oxide. An illustrative amount of the additive is one to two per cent by weight.

BRIEF DESCRIPTION OF THE DRAWINGS The features of the present invention which are believed to be novel are set forth with particularity in the appended claims. The invention, together with further objects and advantages thereof, may best be understood by reference to the following description taken in conjunction with the accompanying drawings, in the several figures of which like reference numerals identify like elements and in which:

FIG. 1 is a schematic representation of a shadow mask in position in a re-etch work stage;

FIG. 2 is an enlarged view of a fragmentary portion of the screen in the re-etch process; and

FIGS. 3 and 4 are the curves used in explaining the effects of certain parameters in the re-etch process.

DESCRIPTION OF THE PREFERRED EMBODIMENT The screen of the black-surround tube may be prepared in either of two ways, that is to say, the black surround pattern may first be developed on the image area followed by the application of interleaved patterns of three different phosphor materials or the phosphors may be applied first after which the areas intervening the phosphor deposits are covered with light-absorbing material. In either event, the shadow mask is used in forming phosphor dots that are smaller in diameter than the apertures of the mask as finally installed in the tube which is also the case for a post-deflection-focus tube that does not feature black surround. The present invention may be completely understood from a discussion of the phosphor printing process which is common to both types of tubes and, therefore, the steps required to apply the black-surround material will not be described.

The technique of applying a multiplicity of phosphor dot triads throughout the image or screen area of a rectangularly shaped color tube is well known to the art and need not be recited in detail here. Briefly, the faceplate section of the envelope, while still physically separated from the conical envelope portion, is chemically cleaned and then coated with a photoresist material such as a water-based slurry of polyvinyl alcohol sensitized by ammonium dichromate. It is also common practice to include one of the three color phosphors in pulverulent form as an ingredient of the slurry. Assume that green phosphor is first to be applied. The slurrycoated faceplate receives its shadow mask and this subassembly is secured in an exposure position in an exposure chamber or lighthouse. A source of ultraviolet light, positioned to simulate the electron gun of the tube in process that is assigned to excite green phosphor, is energized to direct actinic energy through the holes of the mask to the coated image area. The portions of the slurry layer which are exposed are rendered insoluble in water and constitute a latent image of the green phosphor deposits. That image is developed, after the mask has been removed, by washing the screen with water. This leaves deposits of green phosphor in elemental areas distributed throughout the screen or image area. In like fashion, but with slurries that individually have red and blue phosphor ingredients, the red and blue phosphor dots are established. It is, of course, required that the light source, in the application of each of these two additional phosphors, simulate the electron beams of the tube assigned to excite the respective phosphors. This is a well-known photomechanical printing process in wide commercial use in the production of three-color shadow mask picture tubes. In the printing process the shadow mask serves as a pattern to precisely locate and dimension the various phosphor deposits that constitute the desired phosphor triads. To serve in that capacity, the mask is prepared with a dome shape which approximates a spherical section having a rectangular field of apertures or holes, individually smaller than desired in the final form of the mask but accurately dimensioned for use in screening. To achieve the desired final aperture size, the mask is reetched after the screening shall have been accomplished and the remainder of this description will confine itself to the re-etching steps.

Certainly the art is well aware of the general principles of chemical milling by which, for example, a metallic work piece which is soluble in a particular solvent is provided with an aperture pattern by etching. Controls for the etching process are also generally well known and involve such parameters as concentration of the etchant, specific gravity or degree Baume of the etchant, temperature of the etching solution, etching time and the movement of the etchant relative to the work in process. The subject invention entails a unique control of reetching by the use of an extraordinarily unusual value of specific gravity or Baume of the etchant. It has been discovered that this process technique provides unexpected and highly beneficial results in reetching a barren shadow mask, that is, a mask which has no resist or other protective layer to insulate those portions of the mask not to be etched from attack by an etchant applied to the mask to etch away other portions thereof.

By way of reference, re-etching arrangements to which the present invention has especially attractive advantages are described and claimed in copending US. Pat. applications Ser. No. 6,619, filed Jan. 28, 1970 and US. Pat. Ser. No. 45,681, filed June 12, 1970. Both such applications are in the name of Martin L. Lerner and both are assigned to the assignee of the present invention. The first one addresses itself to the use of a densitometer control system, preferably in a multi-stage re-etching apparatus, to control the etch time in one or more of the stages so that the final hole size of the re-etched mask is accurately determined. The other application teaches that loss of wall thickness in the re-etch process may be reduced by positioning the barren shadow mask generally horizontally with its dome section lowermost and directing the etchant from an array of spray nozzles rotatably supported beneath the dome of the mask to be re-etched. The present invention accomplishes, among other things, a still further minimization of the loss in strength or mask thickness attributable to re-etch.

Referring now more particularly to FIGS. 1 and 2, which correspond to like figures of the second Lerner application, the shadow mask 10 is a relatively thin sheet of cold rolled steel of dome-like configuration. Its thickness at the start of the re-etch is about 7.2 mil. The mask 10 is perforated in that it has a field of apertures, individually configured as shown in FIG. 2 which is discussed more particularly hereafter, and is fixed to a relatively heavy frame 11 formed of like material but of significantly greater thickness, such as mils. The frame has mounting springs 12 which normally serve to hold it in position within the faceplate section of its tube but in re-etch the springs serve to removably support the mask within a fixture 13 equipped with rollers 14 which ride rails 15 to convey the work in process through the several stages of the re-etch apparatus. In each etching station there is a stick or linear array 26 of spray nozzles rotatably supported beneath mask 10 and supplied with an aqueous solution of an etchant,

such as ferric chloride, from a source (not mentioned) to the end that the etchant is sprayed vertically upward against the mask and through each of its holes.

A cross-sectional view of the circular apertures of the shadow mask is shown in FIG. 2 where it appears that each aperture comprises a small-diameter opening 10a on the concave surface or face of the mask and a concentric, large-diameter opening b on the opposite or convex side of the mask with interconnecting areas configured to establish generally conically shaped recesses which provide a feathered, reduced-thickness wall section defining the small-diameter openings 10a. This configuration of the mask holes is most beneficial in permitting the holesto be opened or enlarged in reetch while reducing the loss of wall thickness experienced by the mask itself.

The first step of the re-etch operation, as described in the first-mentioned Lerner application, is an oxide stripping and rinse. This is required because it is common practice to blacken the surface of the shadow mask preparatory to its use in screening so that it exhibits the heatdissipation and light-reflecting properties of a black body. This is accomplished by a surface oxidation of the mask but an oxide coating resists the attack of an etchant and therefore has to be removed prior to re-etch. Stripping the oxide layer bares all the metallic surfaces of the electrodes to the etchant in the reetching step. When the mask advances into the reetching station, assuming the position shown in FIG. 1, the etchant is sprayed upwardly onto its domed surface and passes through the small-diameter openings 10a. The movement of the etchant through the holes results in a reaction with the feathered, reduced wall thickness sections and enlarges the apertures of the mask to the end that finally the mask apertures are larger in dimension than the phosphor dots on the screen. Critical control of the ultimate hole size is readily accomplished through the control system described and claimed in the first-mentioned Lerner application.

As thus far described, both the screening and the reetch processes are in accordance with the prior art; the contribution of the present invention comprises utilizing in the re-etch station a substantially saturated solution of the etchant to which the material of the mask responds. In the usual case, where the mask is cold rolled steel, the etchant is an aqueous solution of ferric chloride.

The curve of FIG. 3 shows the variation in process time in seconds, required to achieve a particular enlargement of the mask apertures, with changes in degree Baume. The plot is premised on a change in hole size from 9.7 to 15.4 mils and makes clear that the etching time increases with an increase in degrees Baume. It is for this reason that the re-etch art heretofore has utilized a Baume of the order of 32 to 36 which falls on the generally linear part of the curve and achieves the desired enlargement of hole size in a reasonably short time. So far as the applicant is aware this range of Baume, which is far from a saturated solution, was determined long ago in the etching art, presumably in conjunction with chemical milling practices wherein a resist covered all surfaces of the work in process except such portions thereof desired to be etched. In any event this has been carried over into the re-etch art and is still the recommended Baume range of manufacturers who supply re-etch equipment.

FIG. 4 shows the variation in mask thickness experienced in achieving the enlargement of the holes to a desired dimension, such as 15.4 mils, with change in degrees Baume. Specifically, the thickness tends to increase with degrees Baume. In accordance with the teachings of this invention, a saturated solution of the etchant is employed in re-etch which, while increasing the etching or process time as indicated in FIG. 3, has distinct advantages with respect to process control, minimizing thickness loss of the mask, and smoothness of the re-etched holes. In contradistinction to prior art re-etch practices, the present invention teaches Baume limits in the range of approximately 44 to 49 degrees for an aqueous solution of ferric chloride for use in reetching barren steel shadow masks.

Since the process time increases with degrees Baume, see FIG. 4, the use of a saturated etchant solution improves process control in achieving a desired final size hole. That is to say, tolerances in respect of processing time are relaxed because the change in hole size for a given error in process time is reduced with the satu' rated etchant solution. Of course an equivalent result could be obtained by controlling the process through temperature of the etchant bath, but control predicated upon temperature does not minimize the reduction in mask thickness which is an added benefit in using the process control achievable with increase in degrees Baume.

The mechanism by which the loss in mask thickness is minimized is not well understood but is believed to be based on the following. What actually takes place in re-etch is a chemical reaction of the etchant with the mask in accordance with the following:

2 Fe c1,+F@ a 3 Fe 01 It has been observed that if there is no movement or relatively little movement of the etchant with respect to the mask very little, if any, etching takes place or, in other words, the reaction of equation (1) is attenuated. It is expected that in the absence of relative movement of the etchant and the work in process, a saturation condition is established in which a protective layer of ferrous chloride Fe Cl builds up on the surface under attack and the reaction stops much the same as if that surface were to acquire a protective resist coating. Where the etchant is directed through the holes of the mask, as indicated by the arrow A of FIG. 2, there is a high degree of relative movement and the feathered boundary of the small-diameter hole 10a is attacked with vigorous reaction which opens the hole as required. On the other hand, where etchant approaches the bare blank surface between the holes, as indicated by arrow B, there is little if any relative movement and that which does occur is in the direction indicated by arrow C. In this area of relatively little movement of etchant with respect to the work in process a saturation condition is experienced and a protective surface layer is established. Accordingly, the reaction occasioned by the etchant is localized to, or is preferential in the area of, the feathered boundary portion of the mask which defines the small-diameter openings Illa and is relatively attenuated on the mask surface that intervenes the small-diameter openings 10a. This is a condition of preferential etching in which the holes are enlarged while the wall thickness is reduced very little and is enhanced by the use of asaturated etching solution because the viscosity increases with increase in degrees Baume. I

In addition to the desirable process control and reduction in wall thickness attendant to the practice of the present invention there is the further advantage of improved smoothness of the surface defining the enarged holes of the mask. Here again the reason for this result is a matter of speculation. it is known that cold rolled steel is granular and that the ferric chloride etchant has a tendency to react more vigorously at the grain boundaries than upon the granular surfaces. Consequently, as the viscosity of the etchant is decreased, and this is a condition attendant to the use of low degree Baume, the etchant is better able to penetrate the grain boundaries and react therewith. Where the grain boundaries are attacked, individual grains of the metal may drop out or be released and thus contribute to roughness of the enlarged hole. This may be likened again to a differential etching, where the grain boundaries are more vigorously attacked than the grains themselves. The use of a saturated etchant, which is simply another expression for high degree Baume, provides greater viscosity of the etchant solution with less likelihood that the etchant will penetrate the grain boundaries and a minimization of this phase of differ ential etching.

In other words, three distinct benefits ensue from the use of the saturated etchant, namely, an improved pro cess control, a reduction in the loss of mask thickness, as well as a smoother surface for the enlarged mask apertures. It has been stated above that the invention contemplates that the Baume be in the range of 44 to 49 wherein the mask is formed of steel and the etchant is an aqueous solution of ferric chloride. Best results have been realized at the high end of that range and there is little if any benefit in exceeding 49 Baume because at higher limits the solubility of the solvent is exceeded and a precipitation of ferric chloride is encountered.

A further advantage of the invention has to do with avoiding the presence of loose particles on the mask which may cause difficulty in the succeeding processing steps in bringing the color tube to its final form or even in the operation of the completed tube. It will be recalled that one of the first steps of the re-etch process is the stripping of the surface oxide coating and it is common practice to rinse the mask in water following the stripping step. This rinse has a tendency to produce rust or remnant oxidation that may lead to loose particles on the mask. While ferric chloride reacts with iron and is able to dissolve it, it does not dissolve ferrous oxide and, therefore, the ferrous oxide may accumulate and cause an undesirable loose particle condition in the reach processing of the mask. When the re-etch takes place with a saturated etchant, in accordance with the instructions of this invention, one is able to employ, as an additive to the etching solution, a l or 2 per cent by weight of free hydrochloric acid which does dissolve ferrous oxide and protects against undesirable loose particles. Indeed, any free hydrochloric acid tends to alter the viscosity of the etchant, specifically to decrease it, but this may be accommodated without adverse effect if saturated etchant is used in accordance with this invention.

No claim is here made simply to an etchant of high Baume, such as in the range of 44 to 49, because saturated etchants as such are well known. it is believed,

however, that if such saturated etchants have been employed in the past to etch metals this has occurred in chemically milling work pieces which have a surface resist pattern to protect areas not to be etched from the influence of the etchant. The subject invention, as described above, concerns re-etching shadow masks which have barren surfaces, unprotected by any resist, and which heretofore have been processed with etchant solutions of much lower Baume, usually well below 40 in the range of 32 to 36.

While a particular embodiment of the invention has been shown and described, it will be obvious to those skilled in the art that changes and modifications may be made without departing from the invention in its broader aspects and, therefore, the aim in the appended claims is to cover all such changes and modifications as fall within the true spirit and scope of the in vention.

We claim:

1. in the manufacture of a color cathode ray tube wherein an interlaced pattern of various phosphor materials is screened on an image area by a photomechanical printing process in which said area is exposed to actinic energy through apertures in a metallic mask formed of steel or a steel alloy which is soluble in an etchant comprising an aqueous solution of ferric chloride,

the improvement which comprises enlarging the apertures of said mask while minimizing reduction of mask thickness, after screening said image area with said phosphors, by applying to the bare metallic surfaces of said mask a substantially saturated solution of said etchant having a Baume of approximately 49.

2. The method of manufacturing a color tube in accordance with claim 1 in which said mask is given a sur face oxidation prior to its use in said photomechanical printing process,

and in which said mask is stripped of its surface oxidation prior to the enlarging of its apertures.

3. The method of manufacturing a color tube in accordance with claim 2 in which said apertures of said mask are circular and individually comprise a small diameter opening on one surface of said mask, a concentric large diameter opening on the opposite surface of said mask with an interconnecting recess configured to provide a feathered, reduced thickness wall section around said small diameter opening,

and in which said apertures are enlarged by spraying said saturated solution on said mask to etch away said feathered wall section.

4. The method of manufacturing a color tube in accordance with claim 3 in which said mask has a domeshaped or spherical configuration and is positioned essentially horizontally during said enlarging step.

5. The method of manufacturing a color tube in accordance with claim 4 in which said etchant solution further includes free hydrochloric acid in the amount of about I to 2. per cent by weight. 

2. The method of manufacturing a color tube in accordance with claim 1 in which said mask is given a surface oxidation prior to its use in said photomechanical printing process, and in which said mask is stripped of its surface oxidation prior to the enlarging of its apertures.
 3. The method of manufacturing a color tube in accordance with claim 2 in which said apertures of said mask are circular and individually comprise a small diameter opening on one surface of said mask, a concentric large diameter opening on the opposite surface of said mask with an interconnecting recess configured to provide a feathered, reduced thickness wall section around said small diameter opening, and in which said apertures are enlarged by spraying said saturated solution on said mask to etch away said feathered wall section.
 4. The method of manufacturing a color tube in accordance with claim 3 in which said mask has a dome-shaped or spherical configuration and is positioned essentially horizontally during said enlarging step.
 5. The method of manufacturing a color tube in accordance with claim 4 in which said etchant solution further includes free hydrochloric acid in the amount of about 1 to 2 per cent by weight. 